Technical Field
This disclosure relates to an organic light emitting diode display and a method for manufacturing the same, which can improve an aperture ratio by forming a storage capacitance using a transparent conductive material.
Discussion of the Related Art
Recently, there have been developed various types of flat panel displays capable of reducing the weight and volume of cathode ray tubes, which are disadvantages. The flat panel displays include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), an electroluminescence (EL) device, and the like.
EL devices are classified into inorganic EL devices and organic light emitting diode displays according to materials of emission layers. The EL devices are self-emitting devices and have advantages of a fast response speed, high emission efficiency and luminance, and wide viewing angles.
FIG. 1 illustrates the structure of an organic light emitting diode. The organic light emitting diode, as shown in FIG. 1, includes an organic EL compound layer, and cathode and anode electrodes Cathode and Anode opposite to each other with the organic EL compound layer interposed therebetween. The organic EL compound layer includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL.
In the organic light emitting diode, excitons are formed in an excitation process when holes and electrons respectively injected into the anode electrode Anode and the cathode electrode Cathode are recombined in the emission layer EML, and light is emitted by energy from the excitons. An organic light emitting diode display (OLEDD) display images by electrically controlling the amount of light generated in an emission layer EML of an organic light emitting diode as shown in FIG. 1.
OLEDDs using characteristics of organic light emitting diodes that are electroluminescence elements are classified into passive matrix type organic light emitting diode displays (PMOLEDs) and active matrix type organic light emitting diode displays (AMOLEDs).
The AMOLED displays images by controlling current flowing in an organic light emitting diode using a thin film transistor (hereinafter, referred to as a “TFT”).
FIG. 2 is an example of an equivalent circuit diagram showing the structure of one pixel in an OLEDD. FIG. 3 is a plan view showing the structure of one pixel in the OLEDD. FIG. 4 is a sectional view showing the structure of the OLEDD taken along line I-I′ in FIG. 3.
Referring to FIGS. 2 and 3, the AMOLED includes a switching TFT ST, a driving TFT DT connected to the switching TFT ST, and an organic light emitting diode OLED contacted with the driving TFT DT.
The switching TFT ST is formed at a portion where a scan line SL and a data line DL intersect each other. The switching TFT ST has a function of selecting a pixel. The switching TFT ST includes a gate electrode SG branched from the scan line SL, a semiconductor layer SA, a source electrode SS, and a drain electrode SD. The driving TFT DT has a function of driving the organic light emitting diode OLED of the pixel selected by the switching TFT ST. The driving TFT DT includes a gate electrode DG connected to the drain electrode SD of the switching TFT ST, a semiconductor layer DA, a source electrode SD connected to a driving current line VDD, and a drain electrode DD. The drain electrode DD of the driving TFT DT is connected to an anode electrode ANO of the organic light emitting diode OLED.
More specifically, referring to FIG. 4, the gate electrodes SG and DG of the switching and driving TFTs ST and DT are formed on a substrate SUB of the AMOLED. A gate insulation layer GI covers the gate electrodes SG and DG. The semiconductor layers SA and DA are respectively formed on portions of the gate insulation layer GI, overlapped with the gate electrodes SG and DG. The source electrodes SS and DS and the drain electrodes SD and DD are formed opposite to each other at predetermined distances on the semiconductor layers SA and DA, respectively. The drain electrode SD of the switching TFT ST is contacted with the gate electrode DG of the driving TFT DT through a contact hole formed in the gate insulation layer GI. A protective layer PAS covering the switching and driving TFTs ST and DT configured as described above is coated on the entire surface of the substrate SUB.
Particularly, when the semiconductor layers SA and DA are formed of an oxide semiconductor material, the semiconductor layers SA and DA are advantageous to high resolution and high-speed driving in a large-area TFT substrate with large charging capacitance because of their high charge mobility. However, in order to secure the stability of the elements, the oxide semiconductor layers SA and DA preferably further include etch stoppers SE and DE for protection from an etchant on upper surfaces thereof, respectively.
A color filter CF is formed at a portion corresponding to the region of the anode electrode ANO to be formed later. The color filter CF is preferably formed to occupy an area as wide as possible. For example, the color filter CF is preferably formed to be overlapped with many regions of the data line DL, the driving current line VDD and a scan line SL. Since several components are formed on the substrate having the color filter CF formed thereon, the surface of the substrate is not flat, and many step coverages are formed on the substrate. Thus, an overcoat layer OC is coated on the entire surface of the substrate in order to planarize the surface of the substrate.
The anode electrode ANO of the organic light emitting diode OLED is formed on the overcoat layer OC. Here, the anode electrode ANO is connected to the drain electrode DD of the driving TFT DT through a contact hole formed in the overcoat layer OC and the protective layer PAS.
A bank BN is formed on a region in which the switching TFT ST, the driving TFT DT and the various lines DL, SL and VDD are formed, on the substrate having the anode electrode ANO formed thereon, to define a pixel.
The anode electrode ANO exposed by the bank BN becomes an emission region. An organic light emitting layer OLE and a cathode electrode layer CAT are sequentially laminated on the anode electrode ANO exposed by the bank BN. When the organic light emitting layer OLE is made of an organic material emitting white light, a color assigned to each pixel is displayed by the color filter CF positioned below the organic light emitting layer OLE. The OLEDD configured as shown in FIG. 4 becomes a bottom emission display in which light is emitted downward.
In the bottom emission OLEDD described above, a storage capacitance STG is formed in a space in which the driving TFT DT and the anode electrode ANO are overlapped with each other. The OLEDD displays image information by driving organic light emitting diodes. In this state, a considerably large amount of energy is required to drive the organic light emitting diodes. Therefore, a large-capacity storage capacitance is required to exactly display image information such as moving pictures in which data values are quickly changed.
In order to sufficiently ensure the magnitude of the storage capacitance, the area of a storage capacitance electrode should be sufficiently large. In a bottom emission OLEDD, there occurs a problem in that, if the area of a storage capacitance increases, an area emitting light, i.e., an aperture ratio decreases. In a top emission OLEDD, a storage capacitance can be formed below an emission region. Thus, although a large-capacity storage capacitance is designed, the aperture ratio does not decrease. However, in the bottom emission OLEDD, there is a problem in that the area of the storage capacitance is directly correlated with the decrease in the aperture ratio.